Aircraft: EMBRAER UNDIFFERENTIATED OR OTHER MODEL
Phase of flight: Takeoff / Launch
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown

Attempting to get off the gate in DEN was difficult; frequency congestion made it very hard to get a word in. After pushing back from the gate; we started engine one. Received taxi instructions and proceeded to our spot. Switched to Ground and received taxi instructions to [Runway] 25. At this point; I went heads down and started programming for [Runway] 25. I told the Captain I was ready. Captain called for the Runway Performance Change Checklist. The checklist was completed in full because we were setup for [Runway] 34R. Captain called for the Before Takeoff Checklist. I did my flow including the takeoff configuration; and we finished the checklist. Tower instructed us to 'lineup and wait Runway 25' I read that back to Tower. The aircraft in front was then cleared for takeoff. This had our attention; as it was not standard. Approaching Runway 25; Captain stated 'Runway 25 confirmed'. I turned on all the lights. After lining up with the centerline; the Captain stated 'heading checks'; I repeated 'Heading checks'. Tower cleared us for 'takeoff on 25 RNAV [FIX]'; I read that back. The Captain started to advance the thrust levers and I looked at the ICAS and noticed engine 2 was not started. I stated 'Engine 2 not running' the Captain brought the thrust levers back to idle. I told Tower we were going to turn off the runway. Tower cancelled our takeoff clearance and asked if we needed assistance. I stated 'negative'. We started engine 2; waited 2 minutes; advised ready and flew to [destination] without incident. Distraction during taxi concerning the change of runway. Frustration with the ramp circumstances. Lack of verifying the engine was running on my part before initializing the Before Takeoff Checklist. Distracting circumstances with the takeoff runway. Making a verification of engine status a step in a flow or checklist. As painfully obvious as it may be. Denver was experiencing long pushback and taxi out delays for unknown reasons. The ramp congestion was such I elected to taxi out to the runway single engine. My habitual practice in my 2 years on the aircraft is to start both engines at the gate and assess if an engine should be shut down due to excessive taxi delays. We initially setup for a departure on RWY 34L. Upon taxi instructions we were given a runway change to RWY 25. Once on the active taxiway the First Officer began the runway performance change while I continued taxiing. Upon completion I requested the 'runway performance change checklist' followed by the 'before-takeoff-checklist.' As soon as the First Officer returned from briefing the passengers ATC instructed us to taxi into position and hold. Both the First Officer and I questioned why we did not hear a takeoff clearance for [previous aircraft] who was already accelerating down the runway. This began notable confusion as to why we were being cleared to taxi into position and hold. The sequencing of events was not usual and lead to confusion. Once we were in position for takeoff the Tower issued our takeoff clearance which we accepted. At this point we realized that the number 2 engine was not running. We did not accelerate for takeoff but did notice the engine was not spooling and requested to cancel the takeoff clearance and exit the runway. My election to taxi on a single engine which is was not my habitual technique. The confusion between the crew regarding the ATC clearance to taxi into position and hold. Captain's failure to clarify confusing ATC sequences. The lack of an aircraft equipment to identify proper takeoff aircraft configuration. The lack of a delayed-engine-start or single-engine-taxi checklist. Previous night's rest in hotel was not optimum. 6.5 hour sit in ZZZ before our flight induced low energy and motivation. Adhering to consistent practices. I.E. Dual engine taxi. Implementing a 'single-engine-taxi' checklist that ensures completion of second engine start. Including a checklist item to ensure the delayed engine start has been completed. Implementing technical features to alert crew of takeoff configuration. Although I the Captain take full accountability for the event I do feel there are technical as well as operational measures that can be developed to ensure these incidents do not happen. In my employment at other airlines; when a single engine taxi was performed the checklist challenged the crew of the second engine start. I feel this would have likely prevented this event. ERJ-175 flight crew reported rejecting takeoff after recognizing that the second engine had not yet been started.